This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Electromechanical and optical sensors have been used in the past for monitoring various object characteristics, including vibration associated with industrial machinery and minute displacements of man-made structures. Examples include strain gauges for measuring deformation; accelerometers for measuring vibration, acceleration, velocity and displacement; and laser-based systems that sense the displacement of optically reflective targets. Such devices, although often accurate, require the sensor to be either in direct contact or at close proximity to the object under test.
Optical sensors on the other hand generally require optically reflective surfaces and precise alignment. For non-contact standoff monitoring applications, the current state-of-the-art is almost exclusively based on laser Doppler vibrometry/velocimetry (heterodyne interferometer) that can measure subtle surface motions collinear with the sensor's line-of-sight. The signal-to-noise-ratio (S/N) of an optical sensor is proportional to the square root of the reflected signal from the surface of a target. With most natural and man-made structures having optically rough surfaces, photon scattering often results in little signal being reflected back to the sensor and thus limits the range and sensitivity of optical systems. In addition, optical signals are strongly affected by a wide range of ambient circumstances including atmospheric conditions such as weather, target composition and measurement configuration, for example, incidence angle and line-of-sight accessibility.
With regard to the measurement of natural objects, the ability of microwave energy to penetrate through clothing with little attenuation and its reflection off of human skin has been exploited to detect concealed objects. For example, radar techniques have been attempted for remote detection of human vital signs. However, these tests have been performed at the microwave or lower range of the millimeter-wave band and thus lack certain of the advantages of a millimeter wave measurement system.
The longer electromagnetic wavelengths of millimeter-wave (MMW) techniques can be harnessed to overcome certain limitations associated with conventional optical sensors. Active and passive MMW techniques have been attempted in the past for various remote sensing applications. Millimeter wave techniques have also been used for nondestructive examination of materials, but at close standoff distances.